"You are going to be, in the next several months ... fill in the blanks," David Letterman teased Sheen on Monday's edition of "Late Show."
"Grandfather," Sheen said, describing the news as "fabulous" but admitting he is still trying to adjust to his new title.
"I guess some of you have heard... Charlie Sheen is going to be a Grampa... yes GRAMPA! His beautiful daughter Cassandra is gonna be a mama," TVGuide.com said Sheen's ex-wife, actress Denise Richards, said via Twitter.
Sheen, 47, is best known for his work on the sitcoms "Anger Management," "Two and a Half Men" and "Spin City," as well as the films "Lucas," "Platoon" and "Hot Shots."